Liquid crystal display having a field emission backlight

ABSTRACT

An LCD display has a programmable backlight device that produces multiple different color fields from multiple different phosphor elements. The backlight device can be a low resolution FED device. The phosphor is applied by an electrophotographic screening process or direct electrostatic screening process. The FED device can further incorporate a wide gamut phosphor.

RELATED APPLICATIONS

This application claims the benefit, under 35 U.S.C. §365 ofInternational Application PCT/US2006/38938, filed on Oct. 4, 2006, whichwas published in accordance with PCT Article 21(2) on Jan. 3, 2008, inEnglish and which claims the benefit of U.S. provisional patentapplication No. 60/817,241, filed on Jun. 28, 2006.

This application relates to U.S. application Ser. No. 12/448,297 filedon Dec. 5, 2007 which published as US 2010-0045589A1; Ser. No.12/308,906 filed on Oct. 4, 2006 which published as US 2009-0185110A1;and Ser. No. 12/448,285 filed on Dec. 18, 2006 which published as US2010-0060820-A1.

FIELD OF INVENTION

The invention relates to liquid crystal displays having intelligentbacklighting.

BACKGROUND OF THE INVENTION

Liquid crystal displays (LCDs) are in general light valves. Thus, tocreate an image they must be illuminated. The elementary picture areas(pixels, sub-pixels) are created by small area, electronicallyaddressable, light shutters. In conventional LCD displays, color isgenerated by white light illumination and color filtering of theindividual sub-pixel light transmissions that correspond to theindividual Red, Green, and Blue sub-images. More advanced LCD displaysprovide programmability of the backlight to allow motion blurelimination through scrolling of individual pulsed lights. For example,scrolling can be achieved by arranging a number of cold cathodefluorescent lamps such as the LCD display in U.S. Pat. No. 7,093,970(having approximately 10 bulbs per display) in a manner such that thelong axis of the lamps is along the horizontal axis of the display andthe individual lamps are activated in approximate synchronism with thevertically progressive addressing of the LCD displays. Alternatively,hot filament fluorescent bulbs can be employed and can likewise bescrolled, with the individual bulbs progressively turning on and off ina top-to-bottom, cyclic manner, whereby the scrolling can reduce motionartifacts. Known LCDs which can utilize scrolling can have aconfiguration similar to that shown is FIG. 1. The backlighting lamps 58are positioned before a diffuser 51. Following the diffuser 51 is apolarizer 52 and a circuit plate 53 having address circuits andassociated first surface pixel electrodes on a first glass substrateplate. The device further includes the liquid crystal material (LC) 54positioned after the circuit plate 53. The LCD display also includes asecond glass plate 55 supporting second Surface electrodes, a colorfilter 59, a second polarizer 56 and a surface treatment film 57, asshown and ordered in FIG. 1.

A further improvement to the standard LCD technology can be obtained byutilizing LEDs (Light Emitting Diodes) for the backlights. By arrangingsuch LEDs in a uniformly distributed manner behind the liquid crystalmaterial and providing three sets of LEDs (Blue, Green, and Red) thatcomprise the entire backlighting system, additional programmability andadditional performance gains can be obtained. Key features of such LEDilluminators include superior black levels, enhanced dynamic range, andalso the elimination of the color filter 59 indicated in FIG. 1. Thecolor filter 59 can be eliminated by operating the backlight and the LCDin a color field sequential manner. While LED backlights can provideexcellent image characteristics, their costs are high.

BRIEF SUMMARY OF THE INVENTION

A method for manufacturing a display device comprises the steps of:depositing a conductor on an anode plate; aligning a mask with apertureson the anode plate such that apertures are in predetermined locationsover selected regions on the anode plate; setting electric field linesbetween the mask and the anode plate by applying one voltage to theconductor and applying another voltage on the mask; and forming phosphorelements on the selected regions by injecting a flux of charged phosphorparticles toward the mask, the charge of the charged phosphor particlesbeing selected such that the charged phosphor particles are forced bythe electric field lines to deposit on the selected regions. The methodcan further comprise the steps of aligning the anode plate with acathode plate having electron emitters associated with the phosphorelements and aligning a liquid crystal device with the anode plate suchthat specific elements of the liquid crystal device are aligned withspecific phosphor elements on the anode plate.

Alternatively, the method for manufacturing a display device comprisesdepositing an organic conductor film on an anode plate; depositing anorganic photoconductor film on an anode plate; charging the organicphotoconductor film; discharging portions of the organic photoconductorby selectively exposing the portions to light; applying a flux ofcharged phosphor of one polarity to the anode plate; and terminating theflux when the charged phosphor completely fills its intended targetregions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an existing LCD with backlight lamps;

FIG. 2 is a sectional view of an LCD with multicolor backlightingaccording to the invention;

FIG. 3 is a sectional view of a field emission device used forbacklighting an LCD according to the invention;

FIG. 4 is another sectional view of a field emission device used forbacklighting an LCD according to the invention;

FIG. 5 is a plan view of a plurality of phosphor elements in the fieldemission device according to the invention;

FIG. 6 shows the CIE 1931 Chromaticity Diagram;

FIG. 7 shows a sectional view of the deposition of charged phosphoraccording to one direct electrostatic process (DES) according to theinvention;

FIG. 8 shows a sectional view of the deposition of charged phosphoraccording to another direct electrostatic process (DES) of theinvention;

FIG. 9 shows process steps following the deposition of charged phosphoraccording to one of the direct electrostatic processes (DES); and

FIG. 10 shows a sectional view of the electrophotographic screeningprocess (EPS) of deposition of charged phosphor according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An exemplary embodiment of the present invention will be described withreference to the accompanying figures. FIG. 2 shows a cross sectionalview of an exemplary LCD display having an LCD front end component 60and an FED backlight (or FED backlight component) 50. In the exemplaryembodiment, the individual phosphor elements 33 run in vertical stripesor patched as shown in FIG. 5; however, the invention does includeembodiments where the phosphor elements 33 run horizontally and wherethe phosphor elements of a given color run continuously. FIGS. 3 and 4show different cross sectional views of the FED backlight 50 accordingto the exemplary embodiment of the invention. ID the figures, the Y-axisis the vertical axis and the X-axis is the horizontal axis. As will bedescribed, having the individual phosphor elements permits intelligentbacklighting for the LCD.

The FED backlight 50 has a cathode 7 comprising a plurality of emitters16 arranged in an array that emit electrons 18 due to an electric fieldcreated in the cathode 7. These electrons 18 are projected toward theanode 4. The anode 4 can comprise a glass substrate 2, having atransparent conductor 1 deposited thereon. The individual phosphorelements 33 can then be applied to the transparent conductor 1 and canbe separated from one another. The transparent conductor 1 can be indiumtin oxide. The phosphor elements 33 can comprise red phosphor (33R),green phosphor (33G), and blue phosphor (33B), as arranged FIG. 5.

The operation of the FED backlight 50 involves the electrons 18 from theplurality of emitters 16 in a cathode 7 striking phosphor elements 33 onan anode plate 4 and causing photon emission 46. A grouping of emittercells 27R, 27G, 27B represented in FIG. 2 correspond to individualphosphor elements 33. Potential 15 is applied to the anode 4 duringdisplay operation. To emit electrons 18 from particular array emitterapertures 25, a gate potential Vq is applied to specific gates 26 whichmay be supported on dielectric material 28. As shown in FIGS. 3 and 4, aplurality of gates 26 (and consequently a plurality of emitter cells)can be used in one phosphor element 33.

The dielectric material 28 and electron emitters 16 can be supported ona cathode assembly 31 which can be supported on a cathode back plate 29,which in turn is supported on back plate support structure 30.

The brightness of the FED backlight 50 can be greatly enhanced by thepresence of thin, reflective metal film 21 on the cathode side of thephosphor. Essentially, the reflective metal film 21 can double the light46 observed by the viewer. The reason is that the reflective metal film21 reflects the portion of the light emitted toward the cathode plate sothat upon reflection it propagates away from the cathode 7 toward theviewer.

FIG. 2 shows an exemplary embodiment of the LCD display utilizing theFED 50 shown in FIGS. 3-5. Here the FED backlights 50 are positionedbefore a diffuser 51. The LCD display according to the invention isgenerally intended to include the diffuser 51. Following the diffuser 51is a polarizer 52 and a circuit plate 53. The diffuser 51 and polarizerstack may include additional brightness enhancement elements such as aVikuiti™ optical film made by 3M which increases the brightness ofliquid crystal displays (LCDs) by recycling otherwise unused light (suchas that is absorbed by the polarizer) and optimizing the angle of thelight incident on the liquid crystal. The LCD further includes theliquid crystal materials (LC) 54 positioned after the circuit plate 53.The LCD display also includes a second glass plate 55, a secondpolarizer 56 and a surface treatment film 57, as shown and ordered inFIG. 2. Regarding the emitters 16 shown in FIGS. 3 and 4, they are shownas being conical microtips emitters. However, carbon nanotubes emittersare preferred. Carbon nanotube cathodes can be effective in FEDsoperating at anode potential of 10 kV or greater in the pixel resolutionrange of 1 mm and larger. A low resolution FED with a coarse diffuser 51provides a substantially locally uniform backlight for the LCD display.(Low resolution implies that a specific phosphor element or a specificrepeat unit of phosphor elements are not exclusive to a specific LCDpixel.) A feature of the invention is that the plurality of theindividual colors from the different phosphor elements 33 can passthrough an individual LCD pixel having but one LCD cell, which canprovide white light or green light, red light, blue light, orcombinations thereof when appropriate phosphor elements 33 are activatedand light therefrom is appropriately diffused in the vicinity of the LCDpixel.

A feature of the invention is that the backlight can be a programmableFED structure, which is referred to as being an intelligent backlight.This means that the FED can selectively provide specific colored lightto specific regions on the screen. This is a benefit because the lightis coordinated with the activation and deactivation of the variousliquid crystal cell regions. By the FED backlights being programmable,the LCD can achieve good black levels, wide dynamic range, and blur-freemotion rendition. Furthermore, it is desirable to provide a backlighthaving as large a color gamut as possible. These characteristics aremost conveniently achieved by utilizing a low resolution FED as abacklight wherein the light-emitting phosphor materials incorporatedinto the FED are selected to provide a wide color gamut.

The novel Field Emission Device (FED) backlight surprisingly exhibitsall the desirable characteristics of an LED, but at substantially lowercost. With the disclosed FEDs being programmable, they can operate in acolor field sequential manner. While the FED structure shown in FIG. 2includes a black matrix 39, a commercial quality LCD display with theFED is attainable without the black matrix. (The black matrix will,however, still provide some nominal improvement in black fields and incontrast). Appropriate x-y addressing of the cathode plate allowsprogrammable emission of electrons from a cold cathode, mostconveniently constructed with carbon nanotube (CNT) technology. A keyadvantage of FEDs is that their programmability is achieved with lowvoltage and low current signals applied in an x-y matrix manner to thecathode structure. Furthermore, as a consequence of the inherentnon-linearity of the field emission phenomenon, no active devices areneeded to be incorporated as switches at the x-y junctions. A furtheradvantage of FEDs is that the power source for the emitted light is asimple DC power supply that in this application is preferably operatedin the 10-20 kV range. A suitable FED for intelligent backlights maycomprise 10-1,000 individually programmable rows and approximately thesame number of columns. In the example FED shown in FIG. 5, each columnhas only one phosphor type and the phosphor colors cycle along each row.In this case, the system can have vertical programmability, whereincolumns can be turned on in their entirety. Alternatively, each row maycomprise a single phosphor color. In this case horizontalprogrammability is provided, wherein a row may be turned on in itsentirety. For the backlight according to the invention, suitable pitchesA (in FIG. 5) between the individual phosphor elements 33 can bedictated by the desired performance requirement of the LCD display. Anexample dimension of the pitch A can be several millimeters (e.g., 1-5mm). As shown in FIGS. 3 and 4, an individual phosphor element 33 canhave a plurality of emitter cells each with array emitter apertures 25having opening dimension B, as shown in FIG. 3. Suitable openingdimension B values can be about 10 microns. (The opening dimension B inFIGS. 3 and 4 does not necessarily have to be the same value.) The pitchof emitter cells D can be around 15-30 microns. (The pitch of emittercells D in FIGS. 3 and 4 does not necessarily have to be the samevalue). Regarding the spacing C between the anode plate 4 and thecathode plate 7, it turns out that a spacing C from 1 millimeter toseveral millimeters works very well for the FED in a backlight mode inthe LCD display. Preferably, the spacing C is 1-5 mm, which helps tomaintain a very thin display. The spreading of electrons due to spacecharge and emission angle associated with these spacings turns out tonot be detrimental to the color performance of backlight when the pitchA is larger than about 1 mm. In other words, the LCD has a relativelylow resolution requirement for the backlight when the intelligentbacklighting is used. As such, electron spreading between the anode andthe cathode plates is of no significant concern. The carbon nanotube FEDcan provide excellent light output subject to visible graininess due toemission non-uniformities. In the disclosed device, the undesirableconsequences of such emission non-uniformities are renderedimperceptible through the use of an appropriate diffuser between the FEDbacklight and the liquid crystal device. Preferably the disclosed FEDbacklight is operated in the color sequential mode, thus no colorfilters are required; however, another embodiment of the invention caninclude color filters which could provide an opportunity for narrowercolor wavelength ranges. For example, in an FED backlight employing 300individually addressable rows one could assign 100 of these rows to eachof the three colors—Red, Green, Blue, such that upon activating theappropriate control signals in a time sequential manner at any one timeonly the Red or the Green or the Blue phosphor elements from the anodeplate are lit up. FIG. 5 shows an example array of the FED device inplan view of a hypothetical situation in which blue backlight is desiredat a certain time in several rows of two adjacent colored groupingsrepresented as first block 34 (i.e., Red 33R, Green 33G, Blue 33B andRed 33R′, Green 33G′, Blue 33B′) and green backlighting is desired nextin time in the same rows but the next two adjacent colored groupingsrepresented as second block 35 (i.e., Red 33R″, Green 33G″, Blue 33B″and Red 33R′″, Green 33G′″, Blue 33B′″). Note that in the example shownin FIG. 5, only 6 phosphor elements 33 in a column are shown asactivated at a certain time; however, the LCD can be designed andoperated to have the entire column or fraction thereof in the FEDactivated when such color is needed in a particular region of the screenin the LCD. The ratio of individual columns or rows of the individualphosphor elements 33 to the number of pixels lines of the LCD componentaccording to the invention is in the range of 1:3-1000. In a preferredembodiment, the ratio is 1:100-1000. The ratio being between 1:100-1000is preferred because it requires less individual electrical connections,but yet provides adequate backlight uniformity and programmability.

Another aspect of the invention is the selection of phosphor to be usedin the FED backlight 50. It is desirable to provide the widest possiblecolor gamut. Known FEDs utilize either low voltage phosphor materials orCRT phosphor materials. In the 10-15 kV preferred operating range, CRTphosphor materials are the most suitable. Different phosphor materialsand their characteristics are indicated in the following table.

Type Phosphor x y Im/W Decay (s) NTSC space (%) Comments NTSC GreenZn₂SiO₄: Mn (Zinc Orthosilicate- 0.21 0.71 31 10⁻² 100 Large color gamutand low Magnanese) efficacy CRT Green ZnS: Cu, Al, Au (Zinc Sulfide-0.30 0.62 65 10⁻⁵ 70 High efficacy EBU - skin tones Copper, Aluminum,Gold) Alternative SrGa₂S₄: Eu (Strontium 0.27 0.68 55 10⁻⁶ 85 Goodgamut; high current Green Thiogallate: Europium) saturation; moisturesensitive CRT Blue ZnS: Ag (Zinc Sulfide-Silver) 0.14 0.05 10 10⁻⁵ 100High efficacy; pigment added CRT Red Y₂O₂S: Eu (Yttrium oxysulfate- 0.660.33 16 10⁻³ 100 High efficacy; pigment added europium)

The NTSC phosphor materials provide substantially wider color gamut thando CRT phosphor materials. This is shown in the CIE 1931 ChromaticityDiagram in FIG. 6. But as shown in the table, the efficiency of the NTSCGreen phosphor is less than 50% of the CRT Green phosphor. Furthermore,the NTSC Green has a long decay time. Thus it may result in a motiondelay. Therefore, the TV industry has completely switched from theoriginal NTSC Green to the CRT Green.

An alternative Green phosphor, strontium thiogallate:europium, has beenidentified. The CIE 1931 Chromaticity Diagram shown in FIG. 6 shows thatstrontium thiogallate:europium provides a color gamut exceeding that ofCRT Green and approaching that of the NTSC Green. The negative aspect ofstrontium thiogallate:europium phosphor is that it is moisture-sensitive(i.e., chemically decomposes by water). Therefore, such a phosphorcannot be applied with the standard commercial screening techniques,because they require the use of water. As such, strontiumthiogallate:europium phosphor could not be considered.

However, another aspect of the invention provides the means to includenon-water compatible phosphor. Two novel electrostatic techniques forscreening water-incompatible phosphor are herein disclosed for FEDs. Oneis an electrophotographic screening process (EPS) and the other is adirect electrostatic screening process (DES).

One direct electrostatic process (DES) can be best understood withreference to FIG. 7. The process can begin with the application of aconductor on the glass surface. FIG. 7 shows an unfinished anode plate 4having the conductor 1 on glass substrate 2. The conductor 1 can be madeeither by the deposition of a thin layer of metal (e.g. rhodium) or anorganic conductor. In the case where an inorganic conductor will remaininside the FED structure after subsequent processing and/or finishing,the conductor must be transparent. In case of an organic conductor, onecan bake out the organic conductor in subsequent processing steps; assuch, transparency is not required for such conductors.

Next, the process involves placing an electrostatic mask 12 havingapertures 13 positioned some distance in front of the glass substrate 2.The dimension of the apertures 13 can be approximately the desireddimension of the phosphor stripes (i.e. the width of the 33R, 33B and33G such as shown in FIGS. 2 and 5). Approximately implies the aperture13 dimensions are about +/−20% the size of the target width of thephosphor stripe. A bias voltage between the electrostatic mask and theconductor coating the substrate is then applied. The bias voltage willresult in an electric field indicated by arrows and the associatedletter “E” in FIG. 7. A flux of charged phosphor particles 11 in achamber 20 (which can be a closed box structure having appropriateplumbing for piping in the flux) is then caused to move toward thesubstrate 2 and its associated electrostatic mask 12. As such, the fluxis introduced to the chamber 20 on the side of the mask opposite of theglass substrate. The phosphor particles 11 have a charge similar to theelectrostatic mask. As the phosphor particles 11 approach the mask, theparticles will be guided by the fringing electric fields away from thesolid surface of the mask 12 and through the aperture 13 such that theywill be deposited on top of the conductor 1 underneath the aperture 13.The process steps outlined above are done for a first color phosphor. Toapply other phosphor of different color, another electrostatic mask 12can replace the first mask and be appropriately positioned to apply thenext flux of phosphor particles of another color. Alternatively, theexisting electrostatic mask 12 used for depositing the first colorphosphor can be appropriately shifted to deposit the next color phosphorin the targeted locations for that second color.

FIG. 8 shows an alternative DES arrangement. The fundamental differencebetween the arrangement shown in FIG. 7 and that in FIG. 8 is theinclusion of an insulator layer 3 on top of the conductor 2 on thesubstrate. Fundamentally, the arrangement shown in FIG. 8 functionssimilarly to that shown in FIG. 7, except that here in FIG. 8 thephosphor particles 11 which have a charge and propagate through theapertures 13 of the electrostatic mask 12 will retain their charge asthey deposit on the insulator 3, because they are separated from theunderlying conductor 1 by the insulator 3. With the insulator 3, theprocess steps can be the same as those used Iii the application ofphosphor particles 11 shown and described in FIG. 7.

By the inclusion of an insulator 3, another printing process isfeasible. This other process which involves charging the insulator 3through the apertures 13 of the electrostatic mask 11 with coronacharges. The locations on the insulator that were aligned with theapertures 13 will now have a charge. Phosphor particles 11 having chargecould then be introduced in the chamber and will deposit on the locationof the conductor having the opposition charge of the phosphor particles11.

The inclusion of an insulator 3 can also be advantageously utilized in afixing step. The drawings in FIG. 9 show the various stages of theprocess according the embodiments having insulator and a fixing step.FIG. 9A shows a profile of the anode plate 4 following the deposition ofthree phosphor deposits and removal of the electrostatic mask. FIG. 9Bshows the result of a fixing step. The fixing can occur by applying somesolvent that partially dissolves the insulator, thereby allowing thephosphor particles to sink into the insulator 3 and allowing some of theinsulator material through capillary action to diffuse up and into thephosphor. After the solvent evaporates, the result is that the phosphordeposits are now more strongly secured to the anode plate 4. Fixinghelps to prevent unwanted movement of phosphor particles during afilming step. In a preferred embodiment the insulator is a polystyrenematerial of approximately 5-10 micrometers in thickness and the phosphorfixing is accomplished by spraying the anode plate with a solvent.Following the fixing step, a layer of lacquer film 5 can be applied tothe anode plate, followed by the application a reflective metal film 21as shown in FIG. 9C. After a bakeout step, the resulting structure ofthe anode plate 4 is shown in FIG. 9D, if no black matrix 39 isemployed. If black matrix 39 is used it would be applied some timebefore the application of the reflective metal film 21 and the screenstructure would resemble that structure shown in FIG. 2.

Alternatively, the phosphor elements can be deposited by anelectrophotographic screening (EPS) process, which is generally shown inFIG. 10. The process begins by the application of an organic conductorfilm 41 as shown in FIG. 10A, followed by the application of an organicphotoconductor film 42 as shown in FIG. 10B. The organic photoconductorfilm is then charged, preferably by a corona charger, which isrepresented in FIG. 10C. A mask 43 is then positioned by the screen. Themask 43 has apertures corresponding to the preferred locations of thephosphor elements. The organic photoconductor is then exposed to lightwhich passes through the apertures of the mask 43 as shown in FIG. 10D.Areas of the organic photoconductor exposed to the light are dischargedand regions not exposed remain charged. A flux of charged phosphorparticles 11 in a chamber 20 (which can be a closed box structure havingappropriate plumbing for piping in the flux) is then caused to movetoward the substrate 2 and the charge phosphor particles 11 will thendeposit on the regions of the organic photoconductor discharged as shownin FIG. 10E. (Alternatively, phosphor development can involve depositionof oppositely charged phosphor landing on appropriately chargedlocations of the organic photoconductor.) The steps of corona chargingas shown in FIG. 10C, exposing and discharging as shown in 10D, anddeveloping by the application of charged phosphor particles 11 can berepeated to deposit additional phosphor colors. When all of the desiredcolor phosphor deposits are applied to the anode plate 4, the anodeplate can then be fixed and filmed as represented in FIG. 10F. Followingfilming, a reflective metal film can be applied to the anode plate andthe anode plate can be combined to a cathode plate 7 to complete the FEDdevice. After the finishing of the FED, the FED is then joined to theback of the LCD to provide the backlighting for the LCD. The FED devicesmade according the processes disclosed herein, can be also made for useas direct display devices, which do not involve liquid crystal front endcomponents.

The invention claimed is:
 1. A method for manufacturing a display devicecomprising: depositing a conductor on an anode plate; aligning a maskwith apertures on the anode plate such that apertures are predeterminedlocations over selected regions on the anode plate and such that themask is positioned a distance from the anode plate and does not contactthe anode plate and contents on the anode plate, the mask being anelectrostatic mask for holding a bias voltage; setting electric fieldlines between the mask and the anode plate by applying one voltage tothe conductor and applying the bias voltage on the mask such that theelectric field lines include direct field lines between bottom surfacesof the mask to the conductor; and forming phosphor elements on theselected regions by injecting a flux of charged phosphor particlestoward the mask and through the apertures, the charge of the chargedphosphor particles being selected such that the charged phosphorparticles are forced by the electric field lines to deposit on theselected regions.
 2. The method of claim 1 further comprising: aligningthe anode plate with a cathode plate having electron emitters associatedwith the phosphor elements, and aligning a liquid crystal device withthe anode plate such that specific elements of the liquid crystal deviceare aligned with specific phosphor elements on the anode plate.
 3. Themethod of claim 1, wherein setting electric field lines and formingphosphor are repeated to deposit additional phosphor of different color.4. The method of claim 1, wherein the phosphor particles are strontiumthiogallate:europium.
 5. The method of claim 1 further comprisingaligning the anode plate with a cathode plate having electron fieldemitters associated with the phosphor elements, wherein the displaydevice is a field emission display device.